Chromium-carbon and chromium-nickel-carbon resistive films

ABSTRACT

A resistor which includes a film consisting essentially of 1-15 percent, by weight, carbon and the remainder a material selected from the group consisting essentially of chromium and nickelchromium.

United States Patent Inventor James M. Booe 548 N. Audubon Road, Indianapolis, Ind. 46219 Appl. No. 886,968

Filed Dec. 22, 1969 Patented Oct. 5, 1971 Continuation-impart of application Ser. No. 757,512, Oct. 5, 1968, now abandoned which is a continuation of application Ser. No. 371,765,June l, 1964, now abandoned.

CHROMlUM-CARBON AND CHROMiUM-NICKEL- Primary Examiner-E. A. Goldberg Atwrney.rRichard H. Childress, Robert F. Meyer, Henry W.

Cummings and C. Carter Ells, Jr.

CARBON RESISTIVE FILMS l0 1 Drawing ABSTRACT: A resistor which includes a film consisting essen- U.S. Cl 338/308, tially of 1-15 ercent, by weight, carbon and the remainder a 252/503, 338/332 material selected from the group consisting essentially of Int. Cl 1101c 7/00 chromium and nickel-chromium.

2O 4 s -ll ////I ;;II// r 1 K l6 2 l4 PATENTED 0m SIB?! 3611.246

INVENTOR JAMES M. BOOE A'ITORNEY CHROMlUM-CARBON AND CHROMlUM-NICKEL- CARBON RESISTIVE FILMS This is a continuation-in-part of application Ser. No. 757,512 filed Aug. 5, 1968 which is a continuation of application Ser. No. 371,765, filed June 1, 1964, both now abandoned.

This invention relates to resistors and more particularly to resistors having resistive films and to means and methods for producing resistors having resistive films of superior electrical, mechanical and chemical properties without the use of a vacuum system.

It is known in the art to employ chromium and nickelchromium alloys as resistive films in electrical resistances. The aforementioned materials are particularly noted for possessing a low-temperature coefficient of resistance, load-life stability and reproducibility. In the case of nickel-chromium alloys, resistors are generally made by winding a wire of the alloy on a suitable substrate. This, however, is not feasible in the case of chromium as chromium wire has insufficient ductility. However, recently both nickel-chromium alloys and chromium metal were employed in resistor elements by vacuum evaporation onto suitable substrates. Although this process has certain advantages, particularly in the making of miniature resistance elements, it has the disadvantage of being a time-consuming and expensive process. It is, therefore, highly desirable to employ a process which will achieve approximately the same quality of resistance elements, but to accomplish this in a shorter period of time, at lower cost and without the expensive vacuum deposition equipment required in current art.

An additional disadvantage of nickel-chromium or chromium resistance elements, particularly in the form of very thin films, is their lack of durability, particularly with respect to corrosive agents which may exist in the atmosphere. The main example of this is the presence of salt atmosphere which is particularly prevalent near sea coasts which is commonly known to be highly corrosive to metals. Other examples of corrosive conditions which are commonly known to be deleterious to the metals and resistance elements are contaminates in industrial atmospheres. Wherever a corrosive environment exists, corrosive action is greatly enhanced by the current flow through the element where a large potential exists between the terminals or across the element by virtue of the device. This electrical potential promotes electrochemical corrosion of the resistance film thus changing its resistance value and ultimately corrodes the element into. This mode of failure has been a common occurrence for many years even in chromium resistance films having a low-temperature coefficient of resistance.

It is an object of the present invention to provide a resistor having carbon containing nickel-chromium or metallic chromium resistance films, the resistor having good load-life stability.

It is an object of the present invention to provide a resistor having carbon containing nickel-chromium or metallic chromium resistance films, the resistor having good reproducibility of resistance.

It is an object of the present invention to provide a resistor having carbon containing nickel-chromium or metallic chromium resistance films, the resistor having broad re-' sistance range per unit square.

It is an object of the present invention to provide a resistor having carbon containing nickel-chromium or metallic chromium resistance films, the resistor having a low cost of processing.

It is another object of the present invention to provide a deposition process employing thermal decomposition of organic compounds such as carbonyls and metal dicumenes.

In the drawings, the sole FIGURE is a cross section of a resistor employing the principles of the invention.

Generally speaking, the present invention provides a re-.

the film. The existence of carbon in the films is associated with the method by which the films are produced, that is, by the thermal decomposition of solutions of certain unstable organometallic compounds, such as chromium carbonyl, nickel carbonyl and dicumenechromium.

In the preparation of nickel-chromium-carbon films, the mixed carbonyls of nickel and chromium have been found to best serve the purpose. It is known in the art that deposition agents can be obtained in gaseous form and are generally mixed in low concentration with carrier gases such as hydrogen, nitrogen, argon and so forth, they yield deposits of the metals when the gaseous mixture is brought in contact with a heated article in the absence of air.

Although the known vacuum and vapor methods can produce resistance films, the incorporation of carbon in a Mi- 'Cr or Cr film is not feasible or is very difficult to achieve by the vacuum deposition method. Superior films are economically obtained by employing the metal carbonyls dissolved in a suitable solvent. Examples of solvents which may be employed are benzene and toluene. Both nickel andlchromium carbonyls are slightly soluble in these solventsfThere are numerous compounds which will dissolve the metal carbonyls but some do not produce the desired effects. Although benzene and toluene as solvents yield equivalent films, agents as with the chlorinated hydrocarbons as represented by methyl chloroform undergo abnormal decomposition when a heated article is brought in contact with the solvent to yield a carbon deposit on the surface. The extent of this decomposition may be too great to give satisfactory metal deposits of nickel and chromium, nevertheless such solvents as methyl chloroform can be employed in a more stable solvent such as benzene and toluene in order to yield a higher carbon content in the metal or alloy film. This, therefore, would represent an agent for incorporating an increased amount of carbon in the film.

Chromium carbonyl, Cr(CO) is a colorless crystal compound having a molecular weight of approximately 220 and a specific gravity of 1.77 at room temperature. It has a vapor pressure of 0. 17 millimeters of mercury at 25 C. and 60 millimeters of mercury at C. it begins to decompose at C. yielding chromium metal and carbon monoxide.

C. ,..QCQ LI f=. C I Nickel carbonyl, Ni(CO) is a colorless liquid having a molecular weight of approximately 170 and a specific gravity of 1.318 at room temperature. It has a vapor pressure of 330 millimeters of mercury at 20 C. and a boiling point of 43 C. Nickel carbonyl begins to decompose in the range of l90- 205 C. yielding nickel and carbon monoxide.

Ni(CO), Ni+4CO.

Although there is some disparity between the decomposition temperatures and the vapor pressures of nickel and chromium carbonyls, it is not particularly difficult to obtain codeposits of the two metals in any ratio desired. For resistor applications, the preferred alloy is substantially 80 percent nickel and 20 percent chromium by weight. Since the preferred method of obtaining chromium or nickel-chromium resistance films is by the solution process wherein the heated substrate is subjected to a solution of the carbonyls, the disparity between the vapor pressure and their decomposition points becomes less of a factor because volatilization and decomposition of the carbonyls occurs only at the interface of the solution and the substrate. Since both nickel and chromium carbonyl are soluble in the same type of solvents, it is only necessary to regulate the concentration of the two carbonyls to the desired value and employ a solution temperature below the boiling point of nickel carbonyl (43 C. and to select the proper substrate temperature and exposure time.

Since the extent of solubility of these carbonyls in the usual solvents is quite low (generally under 3 percent), the obvious practice is to maintain one of the carbonyls present in the solution at saturation, then regulate the concentration of the other carbonyl to a specific value to yield the desired metal ratios in the deposit.

It is considered a furtheradvantage in the use of the solution process in contributing carbon to the metal deposit. Due to the high temperature of the substrate upon entering the solution, some decomposition of the solvent such as benzene, toluene, and so forth, will yield carbon to a greater extend than that employed by the gaseous deposition method.

[t has been found that the near optimum processing conditions for producing chromium-carbon film from chromium carbonyl are as follows: Benzene or toluene is saturated with chromium carbonyl and the temperature is brought to the preferred range of 50-70 C. Air is excluded from the container by maintaining an atmosphere of nitrogen or argon gas over the liquid. The resistor substrate, preferably of a material having a low coefficient of expansion, such as alumina or quartz, is heated to 400-800 C. but preferably in the range of 600-700 C. The heated substrate is immediately transferred to the carbonyl containing solution through the protective layer of inert gas at such rate that minimum cooling will be afforded. As soon as the heated substrate enters the solution, deposition of chromium and carbon begins. The time of exposure to the solution will depend upon several factors, but primarily upon the size of the substrate to be coated. Also the duration of exposure to the solution will influence the unit resistance of the film produced. For example, an alumina tube or rod 1% inches long by one-quarter inch in diameter preheated to 600 C. will cool in the solution at such a rate that chromium deposition will cease in approximately seconds. ln other words, exposure to the solution for a longer period will neither be harmful nor beneficial. Resistance films prepared under these conditions will have resistivity values of 25 to 50 ohms in the unconfigured condition. Under the same conditions, but limiting the time of exposure to the solution of 5 seconds, resistance values of 100 to 250 ohms in the unconfigured condition will be obtained. A 3-second exposure time will yield deposited films in the range of 1,500 to 3,000 ohms.

In order to stabilize the resistance film, it has been found necessary to subject it to a thermal treatment for about 1 hour to a temperature in the range of 200 C. to 500 C. in air with the preferred temperature range of 300 C. to 400 C.

Referring nowto the drawing, the resistor of the present invention includes a substrate 10, to which there has been applied a resistance film l2, metallic end caps 14 and 16, electrical lead terminals 18 and 20 and protective cover 24. As shown, the substrate is tubular; however, it may be solid if desired. The substrate is, as previously noted, fabricated of a material having a low coefficient of expansion such as alumnia, quartz, or steatite. The resistive film 12 is of the material just described and is applied to the substrate by the method, also just described. Spiralled grooves 22 are formed in the film in order to provide a resistance path of predetermined length to yield the desired resistance.

End caps 14 and 16 are fitted over film l2 and are secured thereby by virtue of being a tight fit. To complete the construction an electrically insulative cover of a suitable plastic may be molded over the film and end caps.

Resistors having chromium-carbon resistance films prepared by the above procedure have the following properties:

1. Load-life stability: With a chromium-carbon film on a high alumina substrate having a length of 1% inches and a diameter of one-quarter inch and operated at 4 watts, the film undergoes only a 0.3 percent change in resistance in 1,000 hours of operation 1% hours on circuit, one-half hour off circuit). Similar specimens operated at the same wattage but in an ambient temperature of 85 C., the films underwent a change of 1.5 percent in 400 hours of operation 1% hours on, one half hour off). Under these conditions, the reproducibility did not exceed 1 percent and the temperature coefficient remained about +100 parts per million.

2. Reproducibility of resistance measurements resulting from an excrusion from room temperature to 150 C. for one half hour and returning to room temperature produced between 0.01 to 0.02 percent change in the room temperature resistance.

3. Temperature coefiicients: It has been found that the temperature coefficient may be varied according to the temperature of the substrate at the time of immersion in the carbonyl solution. A substrate temperature of 500 C. results in a temperature coefficient of about 200 parts per million. A substrate temperature of 600 C. results in a temperature coefficient near zero. A substrate temperature of 700 results in a temperature coefiicient of about 300 parts per million.

Although the specific findings on load-life stability, reproducibility, and temperature coefficient reported above indicate a high degree of integrity of the chromium-carbon film as a resistance element, it should not be construed that these values are the ultimate, but that with further refinement of the processing methods, better characteristics are entirely feasible.

The above process and equivalent properties can be achieved with the nickel-chromium-carbon films.

In addition to the chromium carbonyl method of preparing chromium-carbon films, another starting material is applicable to this general processing method. A relatively new or ganic chromium compound, dicumenchromium (DCC), has recently been marketed by the Union Carbide Corporation. DCC yields chromium deposits under similar conditions as that used for the chromium carbonyl, and the deposits from this material contain carbon. DCC has a molecular structure of two aromatic rings with a chromium atom sandwiched between. This compound has the empirical formula (Cd-1,9, Cr, a molecular weight of 292, and a chromium content of 17.8 percent by weight.

DCC is a dark brown liquid having a viscosity close to that of ethylene glycol. It has sufiicient vapor pressure to enable its volatilization into a heated chamber containing the articles to be coated. Theoretically, the thermal decomposition of this material does not provide for the decomposition of carbon with the chromium. The theoretical decomposition reaction is as follows:

As in the case of the carbonyls, this agent must be used in the absence of air to prevent an abnormal decomposition. Parts to be coated with chromium are heated to a temperature of 300325 C. and upon contact with the carrier gas such as nitrogen or argon containing a small amount of DCC, a chromium deposit will form on the heated articles such as resistor substrates. The rate of deposition of chromium is dependent upon the concentration of the DCC in the carrier gas and the duration of exposure. Film deposits by this process can be obtained in varying thicknesses from a few Angstroms to 0.001 inches or thicker. Although all films formed from DCC contain up to 1 percent carbon, the solution method as hereinafter described is preferred as superior films are obtained.

Although there are several variations in the so-called oven coating process, such as employing different methods of heating the articles to be coated, the use of a carrier gas, the use of vacuum and so forth, this invention also provides for obtaining chromium-carbon films on insulating substrates to prepare resistors by dissolving the DCC in a solvent and employing the method described for chromium carbonyl. This is by far preferable over the vacuum and vapor methods for DCC as the aforementioned methods yield a maximum 1 percent carbon and the desired amounts are 5-10 percent.

Although DCC is soluble in many organic solvents, the preferred types are the long chain stable aliphatics compounds. N-octadecane is recommended as representing one of v the preferred solvents in producing resistor bodies using DCC,

the DCC is dissolved in N-octadecane or a similar solvent to a concentration of less than 50 percent. Although the solution may be used at room temperature, it is preferable to heat same to some elevated temperature but not much above C. The DCC must never be allowed to come in contact with air and in preparing and using the solution it must be kept under an inert gas such as nitrogen or argon.

The resistor substrates which may be quartz, steatite, alumina and so forth are heated to a temperature of 400 to 800 C. but preferably in the range of 550 to 650 C. and subjected to the DCC solution through the blanket of inert gas. The temperature, size and specific heat of the substrate will influence the rate and amount of chromium-carbon film. Also, the duration of exposure to the solution and the concentration of the DCC therein are variable processing parameters.

Chromium films prepared from dicumenechromium also contain carbon, the amount of which depends upon the processing method. A higher carbon content results from the solution method as compared to the prior vapor or vacuum method and thus the solution method is the desired one.

Like the carbonyl produced films, DCC films are extremely resistant to chemical and electrochemical attack, thus yielding resistance films which are durable and long lasting under adverse conditions. Films produced by the carbonyl or DCC solution process are extremely difiicult to dissolve, even by very strong chemical agents, whereas films produced by the vacuum method are more readily dissolved. Further, the resistance to attack of the carbon-containing resistance films formed by the disclosed methods are many times greater than that of pure chromium or pure chromium-nickel resistance films.

Primarily because of the carbon content which may range from l-l5 percent but is preferably between 5-10 percent in the nickel-chromium and chromium films, the use of such films for variable resistors which require a sliding contact over the surface is feasible. In the case of pure chromium and pure nickel-chromium, there has always been the problem of making good electrical contact by a sliding member because of the presence of an oxide film and also because of the inherent nature of chromium and chromium-nickel to seizure particularly when a metallic rubbing contact is moved over the surface.

This invention eliminates the need of an expensive vacuum system, while providing superior films for fixed and variable resistors. It further embraces the use of chromium and nickelchromium containing carbon in an atomic state of dispersion for use as fixed and variable resistors. Although this disclosure emphasizes the use of chromium with carbon and nickelchromium with carbon since these are the most important systems, the invention would also embrace nickel with carbon and any other suitable metal-carbon combination. The invention further embraces solution methods of producing these elements by the thermal decomposition of organic compounds as the metal carbonyls and dicumenes.

The advantages of resistance elements produced by the manner herein disclosed may be summarized as:

1. Extremely resistant to chemical and electrochemical attack,

2. Thermally stable,

3. Lubricity of the film for variable resistors,

4. Low-temperature coefficient of resistance,

5. Good Load-life stability,

6. Good reproducibility of resistance,

7. Broad resistance range per unit square,

8. Eliminates need for vacuum system thereby providing a quicker method and,

9. Low cost of processing.

It will be understood that this invention is susceptible to modification in order to adapt it to different usages and conditions and such modifications in the specific embodiments above will be readily apparent to those skilled in the art. 1 consider all of these variations and modifications to be within the foregoing description and defined by the appended claims.

Having thus described my invention, 1 claim:

1. A resistor comprising a nonconductive substrate,

a resistive film on said substrate, said film consisting essentially of about 1 to about 15 weight percent carbon, the remainder a metal selected from the group consisting of chromium and nickel-chromium,

metallic caps disposed at opposed ends of said substrate and overlying said film,

electrical leads coupled to said end caps, and

an insulative cover substantially surrounding said film and said caps.

2. A resistor according to claim 1 wherein said metal is nickel-chromium in a ratio of about weight percent nickel and 20 weight percent chromium.

3. A resistor according to claim 1 wherein said film consists essentially of about 5 weight percent to about 10 weight percent carbon, the remainder consisting essentially of said metal.

4. A resistor according to claim 1 wherein said film has a thickness of about a few angstroms to about 0.001 inch.

5. A resistor according to claim 1 wherein said substrate is selected from the group consisting of alumina, quartz and steatite.

6. A resistor according to claim 5 wherein said metal is chromium and said substrate is alumina.

7. A resistor according to claim 1 wherein said substrate is a tube.

8. A resistor according to claim 6 wherein said alumina substrate is a tube about 1% inches long by about one quarter inch in diameter and said chromium-carbon film has a resistivity of about 25 ohms to about 3,000 ohms in the unconfigured condition.

9. A resistor according to claim 8 wherein said film consists essentially of about 5 weight percent to about 10 weight percent carbon, the remainder consisting essentially of said chromium.

10. A resistor according to claim 7 wherein said film includes spiral grooves. 

2. A resistor according to claim 1 wherein said metal is nickel-chromium in a ratio of about 80 weight percent nickel and 20 weight percent chromium.
 3. A resistor according to claim 1 wherein said film consists essentially of about 5 weight percent to about 10 weight percent carbon, the remainder consisting essentially of said metal.
 4. A resistor according to claim 1 wherein said film has a thickness of about a few angstroms to about 0.001 inch.
 5. A resistor according to claim 1 wherein said substrate is selected from the group consisting of alumina, quartz and steatite.
 6. A resistor according to claim 5 wherein said metal is chromium and said substrate is alumina.
 7. A resistor according to claim 1 wherein said substrate is a tube.
 8. A resistor according to claim 6 wherein said alumina substrate is a tube about 1 1/2 inches long by about one quarter inch in diameter and said chromium-carbon film has a resistivity of about 25 ohms to about 3,000 ohms in the unconfigured condition.
 9. A resistor according to claim 8 wherein said film consists essentially of about 5 weight percent to about 10 weight percent carbon, the remainder consisting essentially of said chromium.
 10. A resistor according to claim 7 wherein said film includes spiral grooves. 